How do I prevent my Nexus 5X from going into 'deep sleep' ? Android 8.0.0
I was just referring to recent memory of yours, maybe you installed some battery saving app. Anyway, you can go to Settings > Display > Screen timeout (or Sleep) > Choose long times. Some phones have option "Never".
With regards to battery heating up, this happened to me on a newly bought phone. What I just did was uninstall some of my apps, specially those that keep syncing stuff (or you can just set them to manual sync). Hope that helps.
I don't keep a record of when and what apps I install. Maybe I need to do that in the future. Consequently, I can't answer your question.
However, I did do a factory reset ( based on a recommendation from Project fi) because an app was causing my battery to heat up , and the best thing to do was start from scratch. Prior to that, my phone was updated to Android 8.0.